ok dude here goes. 400 ex front a arms will fit along with the tie rods and ends (need the tie rods and ends) 400 ex shocks will fit you just need to get a relocator bracket (jbracing) not sure about the rear end stuff. check out the 350cc sport forum, PLENTY of info on the 300EX. i know i went to one of the links a few times for info. planning to do the same to my exact same bike as yours when "bill" says its ok
2003 300EX K&N/ no air box lid Supertrapp IDS 2 exhaust
Maxxis Razors 22 front 20 rear
Dynojet jet kit
14 front sproket soon to be 36 rear
Hoping for a FST 330 Big Bore
Between duck tape, WD-40 and cajun ingenuity anything is fixable!!!!!!!!!